Light: a quantum particle

Light Sources All hot objects emit light. Examples: Car in the sun --> `heat waves" = infrared (IR) Burnger on a stove --> infrared and red light Light bulb filament --> all visible light sun --> infrared and visible and ultraviolet (UV) Black Body Radiation A body that absorbs all radiation that falls on it is called black body. Cavity with a small entrance hole Energy entering a small hole in a chamber rattles around until it is absorbed. In reverse, the aperture in a heated enclosure appears as a blackbody source. When a black body is heated, it starts to glow and a characteristic black body spectrum is emitted. A black body spectrum has the following properties: More energy is emitted from hot objects than from cool ones. The peak emission occurs at higher frequencies for hot objects and at lower frequencies for cooler objects. The amount of radiant energy emitted by a hot object at various wavelengths. Each curve peaks at a point where T = constant, which is the Wien Displacement Law. A hot filament and the spectra it emits. As the temperature rises form (a) to (c), the corresponding emission curves shift, as shown in the figure above. The peaks of the curves moe toward the yellow, and the bue and of the spectrum increases in intensity as well. The result is that the color of the filament shifts from cherry red to white hot. Try it! Blackbody Radiation Incandescent Light is created by the glow of hot objects. Incandescent electric light consists of hot, glowing filaments (tungsten filament). Light as Particle Important Terms related to Atoms Element A chemical substance that cannot be broken down further Atom The smallest particle that retains its chemical identity Molcule Any collection of two or more atoms bound together Electron An atomic particle with negative charge and small mass Nucleus The small, massive central part of an atom Proton Positively charged nuclear particle Neutron Electrically neutral nuclear particle Ion An electrically charged atom The Bohr Atom An atom consists of a positively charged nucleus and negatively charged electrons, so that the atom itself is electrically neutral. (Model of the atom according to Rutherford) The idea is that the electron can exist at a distance r1 from the nucleus, or at a distance r2 from the nucleus, or at a distance r3 from the nucleus and so on. As long as the electron remains at one of those distances, its energy is fixed. The electron cannot ever, at any time, be in orbit any place between these allowed distances. An electron in an atom can be in any of a number of allowed orbits, and each allowed orbit is at a different energy. One can see this by noting that one would have to exert a force over a distance to move one electron from one allowed orbit to another, just as muscles have to exert a force to kick up a ball a flight of stairs. The allowed energy levels of an atom occur as a series of steps. The electron in the lowest level is called the ground state, while all energy levels above the ground state are called excited states. In (a) the electron of a hydrogen atom is in its ground state. Assume that an electron is in an excited state as shown in (c). The electron can move to the lowest state, but if it does, something must happen to the extra energy. The energy that is left over when the electrically charged electron moves from an upper state to a lower state is emitted by the atom in form of a single packet of electromagnetic radiation, a particle-like unit called a photon. Every time an electron jumps from a higher to a lower level, a photon moves away at the speed of light. In this sense, light behaves like a particle, though it behaves at the same time as a wave. Electrons may jump between the energy levels shown in (a) and in the process absorb (b) or emit (c) energy in form of a photon. This process is called a quantum leap or quantum jump and is fundamental in nature at the atomic scale. Sequence of events in (b): electron in ground state level atom absorbs a photon electon jumps to an excited energy level This process is responsible for an absorption spectum. For example the white light from the sun shows dark absorption lines due to the cooler gases which surround the sun. The presence of sodium in these gases is indicated by two dark lines in the yellow part of the sprectrum. These dark lines are exactly at the same frequencies where the two bright yellow lines occur in the emission spectrum of sodium. Sequence of events in (c): electron in excited energy level transition to lower energy level emission of photons photon: quantum of light with a distinct frequency This process is responsible for an emission spectrum, which consists of distinct lines of a specific color (frequency). The processes leading to spefic emission lines are shown here: The left panel shows that an atom in the first excited state drops back to the ground state with the emission of a long-wavelength, low energy photon. The emission shows up as a long-wavelength, low-frequency spectral line. The right panel shows an atom in the second excited state that drops back to the ground state with the emission of a short-wavelength, high energy photon. Absorption and Emission of Radiation by an Atom